1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and the like, and more in particular, to the temperature control at the fixing unit where a toner image transferred onto a paper or the like is dissolved and fixed.
2. Description of the Related Art
The fixing unit of a copying machine is a unit where a toner image fed from a photosensitive drum and transferred onto a paper is dissolved and fixed onto the paper by fixing rollers. For this purpose, the fixing rollers are provided with a heater lamp for heating.
The fixing rollers are controlled to be kept at temperature previously set according to the size and thickness of the paper, or to circumstances whether the copying machine is in a ready state or in operation, for example. This control is carried out by detecting the temperature on the fixing roller by means of a thermister or the like, and then by lighting and putting out the heater lamp.
Such a temperature control at the fixing unit as mentioned above makes it possible to bring about improvement of the quality of prints as well as power saving. Therefore, delicate temperature control is desirable.
For this purpose, some machines have recently appeared wherein it is possible to carry out delicate temperature control by inputting temperature-detecting signals from a thermister or the like in the analogue port of a microcomputer as control section and by carrying out the temperature control through software.
In the temperature control through software, however, the fixing unit suffers from abnormally high temperature in a case where software has got out of order by unexpected noises or the like to keep the heater lamp turned on. So a temperature fuse is installed as a means for preventing finally occurrences of fires or the like. But there is a danger of deformations being caused in parts such as gears and the like of synthetic resin to bring about serious, mechanical damages, before the fuse is disabled.
Therefore, it can be thought to use a temperature fuse with a low melting point. Such a fuse, however, is disabled at a low point every time abnormal state as mentioned above is brought about, accordingly, there often arises need for replacement of fuses, which is troublesome.